Add more whitespace to Merge Topics Modal

Summary

There is missing space in the "Merge topics" modal on http://gdk.test:3000/admin/topics.

  1. Between both Topic Selectors
  2. Above the alert

Steps to reproduce

  1. As an admin, go to http://gdk.test:3000/admin/topics
  2. Create two topics
  3. Click on the button "merge topics"
  4. Select two topics

What is the current bug behavior?

Not enough whitespace

What is the expected correct behavior?

Enough whitespace, preferrable the one defined in GitLab UI through GlFormGroup

Relevant logs and/or screenshots

Screenshot_2024-08-05_at_14.11.51

Screenshot_2024-08-05_at_16.05.05

Output of checks

Possible fixes

Use GlFormGroup